The control of industrial processes, for example in the chemical and pharmaceutical industry, in the textile industry, in the food- and beverage industry, in the processing of paper and cellulose or in the water purification and waste water treatment is based on the measurement of process parameters which are determined with suitable measuring probes or sensors.
According to reference [1], “Process Measurement Solutions Catalog 2005/06”, Mettler-Toledo GmbH, CH-8902 Urdorf, Switzerland, pages 8 and 9, a complete measuring system consists of a housing, a measuring probe, a cable and a measurement converter (also called a transmitter). By means of the housing, the measuring probe is brought into contact with the process that is to be measured or monitored, for example by immersing the probe in the process material and holding it there. The measuring probe serves to measure specific properties of the process. Through a cable which in the case of reference [1], page 8, has five leads the measuring signals are sent to the transmitter which, in turn, communicates with a process control system and converts the measuring signals into readable data. The measuring probes are selected depending on what properties of the process material need to be measured.
Reference [2], “Prozessanalytische Systemlösungen für die Brauerei” (Process-Analytical Systems Solutions for the Brewery), a company publication of Mettler-Toledo GmbH, CH-8902 Urdorf, Article No. 52 900 309, printed August/2003, describes as an example that in individual stages of the process chain of a brewery (consisting of the water purification stage, the brew house, the fermentation- and storage cellar, the filtration, carbonization and bottling, as well as the waste water treatment) measurements of electrical conductivity, dissolved oxygen, pH value, CO2 value, and turbidity of the process material are performed by means of appropriate measuring probes.
An important factor for a problem-free process control is the condition of the measuring probes, whose properties will normally change over a longer operating time period.
A method disclosed in reference [3], EP 1 550 861 A1, serves to determine the condition of measuring probes which are integrated in a system with one or more stages and which are cleaned from time to time in state-of-the-art CIP- or SIP processes, i.e., without uninstalling the probe. According to this method, the temperature of the measuring probe or of the medium surrounding the measuring probe is measured by means of a sensor that is located inside or outside the measuring probe, and the condition of the measuring probe is determined from the time profile of the measurements of the temperature and in some cases of the process-related value (for example pH) that has been recorded during the operation of the measuring probe.
According to reference [4], WO 92/21962, the hydrogen ion concentration in liquids, i.e. the pH value, is often measured with glass electrodes. Preferably the condition of the glass electrodes is continuously monitored, as the measuring accuracy could become compromised for example if the ion-sensitive membrane is damaged, the diaphragm is contaminated, and if an electric connection inside the electrode is interrupted and/or short-circuited.
According to reference [4], a square pulse that is variable in amplitude and duration is applied with a high impedance to the measuring probe which contains a glass electrode as measuring electrode and also contains a reference electrode; the voltage across the measuring probe which has been changed by the probe impedance is measured and the measured values are compared to a reference value for a new measuring probe that has been determined by experiment or calculation. The square pulses in this arrangement are delivered by an analog output terminal of a processor and sent to the measuring probe by way of a separate conducting lead.
In the method described in reference [5], EP 0 419 769 A2, the monitoring is performed by means of symmetrical bipolar current pulses which are produced by a control unit. The duration of the current pulse periods is freely selectable and can be set to different lengths depending on the accuracy desired for checking the probe. This method requires a comparatively complex circuit, in particular two control leads which, for the purpose of generating symmetrical bipolar current pulses, allow switching between a positive voltage source and a negative voltage source, or switching from the measuring phase in which the pH value is measured to the checking phase in which the electrodes are checked.
A method disclosed in reference [6], EP 0 497 994 A1 relates to the checking of a pH-measuring probe which contains an auxiliary electrode in addition to the glass electrode and the reference electrode. Furthermore, there are two processing devices which are supplied, respectively, by a first and a second generator with an AC test voltage. The first generator in this arrangement operates with a frequency that is an integer multiple of the frequency of the second generator. This allows a separate monitoring of the glass electrode and the reference electrode. In the first case, the property being checked is the resistance of the chain formed of the glass electrode and the auxiliary electrode, while in the second case the resistance of the chain formed of the reference electrode and the auxiliary electrode is being checked. With the selected ratio between the frequencies produced by the generators, a sufficiently accurate differentiation is possible between the output signals in the two processing devices, as in each case one of the output signals is suppressed by the phase-sensitive rectification in the processing device of the other of the two electrodes. The processing devices are therefore no longer directly seeing the difference between the potentials of the glass electrode and the auxiliary electrode. Rather, they detect a difference between the potentials of the glass electrode and the auxiliary electrode or between the potentials of the reference electrode and the auxiliary electrode. As both of the differences in the potentials are referenced to the same potential of the auxiliary electrode, the potential difference between the glass electrode and the reference electrode can be determined with a differential amplifier. In this measuring circuit arrangement, the measuring probe therefore needs to be supplied with the AC test voltages of two different generators. These AC test voltages, in turn, are used for the subsequent phase-coherent processing of the signals and therefore have to be transferred through appropriate conductor leads which normally run from the processing unit to the measuring probe.
However, using additional conducting leads for transferring signals makes the design commensurately more expensive. Furthermore, in systems that are already installed, the required wiring does not exist and can hardly be retrofitted, or only at very high cost and by interrupting the operation of the equipment. This is also a disadvantage because with the trend towards miniaturization and the possibilities that it offers for a decentralized arrangement of intelligent components, the need for transmitting additional signals will rather increase, and more highly developed measuring probes designed for decentralized installation will therefore have only limited use in existing systems.
As was described above, in larger plants as for example breweries a large number of measuring probes are used. Thus, the way in which the installed measuring probes are administrated by the user is of high importance.